One of the most interesting recent developments in survivable network architecture is the method of p-cycles. p-Cycles, or pre-configured cycles, introduced in 1998, are in a sense like BLSR rings, but with support for the protection of straddling span failures as well as the usual protection of spans on the ring itself. The most striking property of p-cycles is that they retain ring-like switching characteristics (only two nodes do any real time switching and are fully pre-planned for each failure) but can be designed with essentially the same capacity-efficiency as a span-restorable mesh network. Recent work has even found that with joint optimization of working path routing along with p-cycle placement, overall designs can approach the ultimate theoretical efficiency levels for any known class of restorable network and having the benefit of dynamic, failure-specific, path restoration. This means p-cycle based networks can be 3 to 6 times more capacity-efficient than ring-based networks while still providing BLSR ring switching speeds. In fact for straddling span failures, the average protection path has half the number of hops of the corresponding ring, so it may even be faster on average. Since 1998, the basic theory of p-cycles as pre-configured structures in the spare capacity of a mesh network has been developed, and there have been studies on self-organization of the p-cycle sets, application of p-cycles to the MPLS/IP layer, application to DWDM networking and studies on joint optimization of working paths and spare capacity. Notably, in one study it was found that full survivability against any span cut could be achieved with as little as 39% total redundancy. This greatly motivates continuing work and practical applications of the p-cycle concept.
Concurrent work on cycle covers under the coincidentally similar name of protection cycles should not be confused with p-cycles. The fundamentally important difference is the aspect of straddling spans in p-cycles. Straddling spans on a p-cycle can each bear two units of working capacity per unit of p-cycle capacity and they require no associated protection capacity on the same spans. All forms of ring or cycle covers fundamentally involve equal (or greater) amounts of protection and working capacity on every span and at best (which is what oriented cycle double covers in accomplishes) reach a 1-to-1 ratio between these, for 100% redundancy. In contrast p-cycles, due ultimately to the effectiveness of straddling span protection aspects, yield fully restorable architectures at well under 100% redundancy.
However, all work so far on p-cycles has been done on what can be called “span-protecting” p-cycles. Each such p-cycle protects only spans that are part of itself or that directly straddle the respective p-cycle. This disclosure extends span-protecting p-cycles to path-segment protecting p-cycles and addresses the issue of mutual capacity (which is intrinsic to any path oriented or multi-commodity flow type of recovery scheme) as well as the corresponding operational complexity in coordinating which paths can access which p-cycles. By extending the concept to path-segment protecting p-cycles, protection against node loss for transient flows by the span-protecting p-cycles is also provided, while flows originating or terminating at the failed nodes cannot be restored by any network rerouting technique. A method for simplifying the task of optimizing the network is also disclosed, the method also making it easier to solve the joint optimization problem.